1. Field of the Invention
The present invention relates to an electronic endoscope having a solid state image pickup device at a front end of an insertion portion thereof. The pickup device converts an image viewed through the endoscope into electrical signals which can be sent to an external device to reproduce the image on a monitor, etc.
2. Description of Related Art
It is necessary to carry out a gamma correction of image signals supplied from a solid state image pickup device, provided at a front end of an insertion portion of an endoscope to reproduce a clear and natural image.
To this end, in a known endoscope, an output value which has been subjected to a gamma correction, corresponding to image signals (original signals) to be input to a gamma correction portion is written in advance in a look-up table in a memory. The image signals are applied as an address signal for the memory of the look-up table to produce a corresponding gamma-corrected value. The corrected signals are then reconverted to analog signals, processed and displayed on a monitor (see for example, Japanese Unexamined Patent Publication No. 1-178235).
However, the gamma-corrected value written in the look-up table has a greater value corresponding to the magnitude of the input original signals. Consequently, the look-up table requires a large memory size for data storage. This results in a substandard memory utilization efficiency and necessitates large and expensive circuits, thereby leading to increased manufacturing costs and power consumption.
Moreover, in the gamma correcting portion, when the original signal is amplified to obtain corrected signals, a noise component is superimposed on the original signal. Consequently, the S/N (signal-to-noise ratio) of the input original signal shown in FIG. 26, deteriorates particularly in a low brightness (dark) range having a high gain, as shown in FIG. 27.
It is an object of the present invention to provide an electronic endoscope in which a gamma correction of an image signal can be effected without deteriorating the S/N.
Still further, in a nonlinear operation with reference to the look-up table, data which is not written in the look-up table, such as data 1-4 between the memory addresses 0 and 1, shown in FIG. 35, is produced.
Consequently, there are large amounts of data which are not written in the look-up table in a low brightness range (dark area) in which data varies considerably depending on the address. Accordingly, a quantization error inherent in the operation of digital data occurs during the gamma correction.
It is a further object of the present invention to provide an electronic endoscope in which a gamma correction of digital image signals can be carried out without deteriorating the S/N or increasing a quantization error.